The anionic extruder polymerization of styrene and other ethylenically unsaturated monomers, particularly 1,3-alkadienes, with an alkyl- or aryllithium initiator is known in the art. In U.S. Pat. No. 3,703,567 a continuous, solvent-free extruder process is disclosed for the production of segment copolymers. A prepolymer of a vinyl aromatic compound and a 1,3-alkadiene is produced in the presence of a catalytic amount of a lithium initiator is fed to an extruder with additional vinyl aromatic compound and 1,3-alkadiene. The temperature of the polymerization in the extruder is kept below 100.degree. C. by the use of a cooling medium circulating through the extruder jacket. In order to stabilize the resulting polymer, a terminating agent and a stabilizer is added during passage through the last quarter of the extruder. In U.S. Pat. No. 3,780,139 a similar process is disclosed wherein vinyl aromatic compound and 1,3-alkadiene are introduced to the extruder with the lithium initiator and during polymerization the temperature is maintained in the range from 50.degree. C. to 150.degree. C. by cooling. The product is a random copolymer.
In extruder polymerizations of these types, the regulation of the polymerization temperature is important because at unduly high temperatures a series of complex reactions takes place which includes thermal termination of polymer growth. This reaction, termed "thermal die-out", may be caused by elimination reactions which produce unsaturated compounds. See Kern et al, J. Appl. polymer Sci., 16, 3123-31 (1972). In general, the rate of termination increases sharply with temperature so that anionic polymerization is difficult if at all feasible at temperatures of 200.degree. C. or above. Polymerization temperatures above 200.degree. C. generally result in adverse effects on the polymer molecular weight and molecular weight distribution.
Most commercial extruders have large shaft diameters to increase throughput of the extruder. Such extruders have limited cooling capacity and temperatures above 200.degree. C. are often observed. Anionic polymerization in such extruders does not provide acceptable product unless extensive and costly cooling is provided.
One solution to this problem is to produce a prepolymer outside the reactor and then introduce the prepolymer with additional monomer feed to the extruder. See U.K. patent No. 1,302,069. By this two-step process it is possible to conduct anionic polymerization in the extruder without undue thermal die-out if polymerization temperatures above about 200.degree. C. are reached. It would be of advantage, however, to provide an improved one-step extruder polymerization process to anionically polymerize ethylenically unsaturated monomers to polymer products of desirable molecular weight and molecular weight distribution.